A baby girl born in Hong Kong was discovered to be pregnant with twins, according to the case study published in the Hong Kong Medical Journal this month.

The nearly 9-pound baby girl, born to Chinese mainland parents, had a rare condition called fetus-in-fetu, which happens when a partially developed fetus becomes incorporated into a normally developing fetus (its twin) in the womb, The Mirror reported. After doctors discovered the growth, the newborn had to undergo surgery in Queen Elizabeth Hospital at just three weeks old to remove them.

"It was almost impossible to detect during the prenatal check-up, as the embryo inside the baby was too small," said Yu Kai-man, a specialist in obstetrics and gynecology.

"Since it is impossible for the little girl to have conceived the pregnancy on her own, the fertilization of the twin fetuses, of course, belongs to her parents, which has gone to the wrong place," the former professor at Chinese University of Hong Kong said.

Initially, doctors were unable to figure out an alien growth between the girl's liver and left kidney, according to KDSK.com. But soon, they discovered that the growth contained what could be "absorbed" siblings, a condition that affects about 1 in 500,000 births.

In other words, the newborn was carrying its own siblings inside the fetus with all three sharing the same DNA, lending support to a popular theory that the woman was initially pregnant with identical triplets.

Between the two partially formed fetus-like structures, one weighed half an ounce and the other weighed a third of an ounce, with both fetuses having legs, arms, a spine, rib cage, bones with bone marrow, "primitive" brain matter and intestines.

They were also covered in skin and each had an umbilical cord, according to the study.

"Judging by the body parts they found in the baby girl, the doctors determined that the absorbed twins could have been up to 10 weeks old before their growth stopped and they were absorbed," ABC News reported. "But the mechanism for this 'absorption' is not fully understood, the study authors wrote, so it's also possible the extra fetuses were absorbed earlier and grew with the baby girl in the womb."

Eight days after the successful surgery, the newborn girl was discharged from the hospital after her "dead" siblings were removed from inside her.

Meanwhile, the World Health Organization has suggested that fetus-in-fetu does not refer to "absorption" of siblings, as is commonly believed, but is a type of cancer called mature teratoma, which is possibly linked to the mother having multiple abortions.

"The widespread use of antenatal ultrasound in early gestation may provide more concrete evidence ... and shed light on this intriguing condition," the report concluded.

Till date, there have been 200 documented cases of the rare condition.